The messages signed by the Syrian Arab Army also urged the rebels to surrender their weapons and warned the countdown to evict foreign fighters has begun. The texts appear to be part of the regime’s psychological battle against the rebels, but are highly unlikely to have any effect on fighters intent on toppling Assad.

Syrians say they began receiving the messages a day after rebels bombed a military command center in Damascus — a major security breach of the heavily guarded capital that highlighted the regime’s growing vulnerability in the face of a rebellion growing in confidence and capabilities.

People with cellular subscriptions received the messages while those with prepaid phones did not, residents in the Syrian capital said.

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Syria’s two cell phone providers are privately owned, and the government would presumably have to pay for the service. Government officials were not available for comment.

In August, Syrian army helicopters dropped leaflets warning rebels in Damascus to hand over their arms and seek amnesty.

Despite Wednesday’s high-profile attack, the two sides have been locked in a stalemate after 18 months of conflict. Activists say the death toll since the uprising against Assad began in March last year has topped 30,000, with nearly two-thirds of the casualties reported in the past five months.

On Thursday, rebel fighters launched a new push to try to take control of Aleppo, Syria’s largest city and a major strategic prize in a stalemated civil war, opposition activists said.

Heavy clashes erupted Thursday in more than a dozen locations in the city, Syria’s largest with 3 million people, said Mohammed Saeed, a local activist. Government forces shelled several districts, he said, speaking via Skype.

Rebels also attacked a compound of Syria’s political security department in the city, said Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, a Britain-based group of activists.

Opposition fighters took control of several neighborhoods in the city two months ago. Since then, neither has been able to dislodge the other.

In Geneva on Thursday, the United Nations’ refugee agency said the U.N. and its partners are launching a revised appeal for $487.9 million to help support Syrian refugees. It said 2,000 to 3,000 refugees are crossing into neighboring countries each day.

The UNHCR said 294,000 Syrian refugees are registered or awaiting registration in neighboring countries.

Earlier Thursday, Lebanese TV station Al-Manar broadcast footage showing Syrian government troops driving rebels from a building they had briefly occupied Wednesday after a double car bomb attack struck army headquarters in Damascus.

The attacks further demonstrated the rebels’ ability to penetrate the tight security in the capital.

Al Manar’s correspondent Youssef Shaeito said he accompanied Syrian troops on Wednesday as they stormed the compound after the explosions. In an interview with the station, he said he saw the bodies of three gunmen inside, suggesting clashes between government forces and rebels inside the building.

The station is owned by the Lebanese Hezbollah militant group, a strong ally of Assad’s regime. Syrian state-run media, however, reported only on the car bombs, omitting any mention of clashes inside the building.

Exclusive Al Manar footage showed a group of soldiers surrounding a building at the compound and shooting at targets inside. Shortly afterward, a pickup truck with a heavy machine gun mounted on it joins the battle.

Eventually, dozens of troops are seen storming the building and going up the stairs from one room to the other amid smoke. The bodies of three gunmen are seen on the floor, one with an apparent suicide belt tied around his waist.

Shaeito said that in the mayhem following the explosions, gunmen had infiltrated the army command headquarters, taken up positions and begun shooting at soldiers. He said the gunmen “moved from floor to floor and room to room” and tossed grenades at soldiers outside.

He said it took soldiers about 45 minutes to storm the building.

International diplomacy has failed to stop the bloodshed in Syria. Syrian activists said more than 305 people were killed across Syria on Wednesday, making it one of the deadliest days since the conflict began.

They included at least 40 people whose bodies were discovered in a suburb of Damascus, some of whom appeared to have been killed execution-style. The exact circumstances in which the victims were killed remain unclear because of strict restrictions on the media.